1. Field of the Invention
This invention relates to a process for the preparation of pre-melted calcium aluminate for iron and steel treatment and protection by the fusion of aluminum dross residues, usually referred to as non-metallic products (NMP) containing variable levels of aluminum (Al) and aluminum nitride (AlN) with calcium carbonate at high temperature, particularly superior to 1400° C., under oxidizing atmosphere such as air and/or CO2. It further relates to the purification of the molten calcium aluminate from some contaminants present in the dross residue.
2. Description of the Prior Art
Aluminum dross is formed whenever aluminum or aluminum alloy is melted and held in the liquid state, during alloying and refining treatment processes and casting under oxidizing atmosphere. Aluminum dross is also formed in larger quantities during the remelting of process scrap and the recycling of post-consumer aluminum products.
Dross must be removed from the liquid surface before casting and rapidly cooled to prevent further oxidation of the aluminum, using various techniques well known by the experts in the field.
The removed dross contains large quantities of metallic aluminum, intimately mixed with oxides of various natures, mainly alumina, magnesia in isolated or combined form such as spinel, MgAl2O4, and with other smaller quantities of various oxides depending on the type of alloys, such as oxides of iron, silicon, manganese, titanium, vanadium, copper, chromium, etc.
Aluminum dross, particularly when originating from the primary operations also contains others impurities such as chlorides and fluorides salts of mixed sodium, potassium, lithium, calcium and/or aluminum composition, originating from the electrolytic reduction and the cast house refining processes. These salts are either carried over from the reduction cells, or formed in the casting furnaces during the refining operations, where they are collected with the aluminum dross during the skimming process.
Since the metallic aluminum content of the dross represents a substantial economic value, it is desirable to recover the maximum possible aluminum content from the dross. The residue left after the aluminum separation from the dross is a mixture of Non Metallic Products, called NMP, consisting essentially of aluminum oxide and nitride with variable quantities of other oxides, chloride and fluoride salts of alkaline and alkaline earth metals and variable residual content of metallic aluminum.
Several processes have been developed to recover the aluminum from dross. For example, dross treatment using various electrical technologies using direct or indirect plasma arc or resistance heating have been proposed, such as described by Dubé et al, U.S. Pat. Nos. 4,959,100 and 5,997,476; Drouet, U.S. Pat. No. 5,245,627; Lindsay, U.S. Pat. No. 4,997,476; or Montagna, U.S. Pat. No. 3,999,980.
Alternatively, the recovery of aluminum metal from solid dross using oxy-fuel heated rotary furnaces or converters in the absence of salt, such as the method described in the U.S. Pat. No. 5,527,380 have also been proposed for achieving an efficient aluminum recovery. For this reason, the NMP generated by these salt-free technologies are relatively clean from salts. The NOVAL non-metallic product from the plasma process (appellation by ALCAN, described in: Recycling of Metals and Engineered Materials, TMS, pp. 1,183, (2000) has been extensively utilized. Because NOVAL-type NMP contain variable level of reactive residual Al and AlN, its energy content is very high, in the order of 4,500 kJ/kg, which makes it particularly well suited for utilization in a high temperature process application.
However, most of the dross in the aluminum industry is being generated by the secondary melting operations and by the recycling of post-consumer scrap, such as used beverage cans. Most of these recycling technologies rely on the utilization of salt flux in order to obtain a high aluminum recovery. Also, primary dross is still often processed using these salt based processes, even if this type of dross contains no or very small quantity of salt. Formerly, the residues obtained from these salt based processes, which are in reality a mixture of salt slag and NMP were disposed of in landfill as waste. Such disposal is increasingly facing environmental restriction or is even banned in a large number of countries. Technologies are under development to separate and reuse the salts from these salty residues using water leaching based processes, such as described in U.S. Pat. No. 5,227,143. Under these conditions the water leached NMP obtained have much lower energy content due to their low Al and AlN content. In some cases, their residual salt level is still high due to the presence of water insoluble salts such as fluoride compounds that are always present at variable concentrations in the smelter dross. For this reason, their reutilization must take this into account. The NMP marketed under the commercial name SEROX, by the firm ALSA (Aluminium-Salzschlacke Aufbereitungs GmbH, Lunen, Germany) falls in this category.
In addition to its utilization in high temperature refractory and concrete industry, basic synthetic calcium aluminate product is extensively used in the iron and steel industries for different metallurgical applications, such as sulfur removal and slag modification and control for ladle metallurgy operations. A key feature highly required by the steel metallurgists is the slag's ability to melt readily in contact with the liquid steel. It is now fully recognized that dense, completely pre-melted calcium aluminate products offer significant advantages in this respect, compared with more porous sintered product material produced by solid state sintering by holding under the melting range of the product, in the range of 1200-1300° C., as opposed to a pre-melted product which is produced by completely melting the product in a large furnace, at temperature above 1400-1500° C. In addition, pre-melted products are much more chemically and physically stable, with low LOI and higher density and less dusty than the sintered material.
The composition of the calcium aluminate desired by the steel metallurgists depends on the specific utilization conditions, but is usually close to the 50/50 mass ratio of alumina to calcium oxide, corresponding to the mayenite main phase Ca12 Al14O33, with an eutectic temperature of about 1375° C.
Pre-molten calcium aluminate is normally prepared by the fusion of high quality bauxite, low in Fe, Si, Ti, and V, with lime in large electric arc furnace. Due to the high costs of the raw material and energy, this product is expensive, and its utilization is normally restricted for special applications in high quality concrete and ceramic applications. For this reason, there is a need to prepare calcium aluminate of high purity, in large volume and at lower costs than presently available for utilization by the steel industry.
The review of the prior art in this field reveals a few attempts to fill this need, which to our knowledge have not been successfully transferred to industrial scale for cost and quality reasons which follow. Most of these processes, such as Breault et al, U.S. Pat. No. 5,407,459 produce sintered calcium aluminate by heating a mix of NMP and limestone below the melting temperature of the calcium aluminate in a large rotary kiln, heated by fossil fuel. As explained by the authors, the significant energy released from the oxidation of the Al and AlN present in variable quantity in the NMP makes the control of the sintering temperature zone close to the melting zone very critical.
In another similar invention, Pickens et al, U.S. Pat. No. 6,238,633, the dross residue was digested in water in order to decompose its Al and AlN content before sintering a mixture of the residues with calcium oxide precursor at temperature below the melting range of the calcium aluminate. The process was reported to be easier to control to avoid the formation of calcium aluminate agglomerate. However; the energy content of the Al and AlN was lost.
More recently, J.M. Iglesias (US 2011/0293494A1) proposed another similar process to generate solid calcium aluminate from the waste generated by the treatment of secondary saline dross through cold sintering or hot sintering between 1100 and 1400° C.
In addition to the fact that these solid state processes produce sintered calcium aluminate which is considered a low grade product for the steel application, they also produce calcium aluminate material which is contaminated by the residual oxides present in the NMP, such as SiO2, Fe2O3, TiO2, V2O5, MnO, Cr2O3, etc. that are known to be detrimental to the steel metallurgy by increasing the slag oxygen potential and/or the risk of steel contamination. As demonstrated by U.S. Pat. No. 6,238,633 this severely limits the type and quantity of NMP that can be utilized to produce the calcium aluminate.
Gens (U.S. Pat. No. 5,135,565) and Kemeny et al (U.S. Pat. No. 5,385,601) describe another approach both using transferred arc plasma device operating under inert gas cover to recover the aluminum content of the dross, not the residue, while producing a liquid slag containing calcium aluminate with a very high level of silica (15%). The slag obtained by Gens contained on the other hand a very high level (8%) of mixed salts (NaCl—KCl) with low levels of calcium aluminate (89-90%). In both cases, the level of impurities in the calcium aluminate is too high to be of any use for the steel industry. This can be explained because these two processes have been primarily designed for recovering the aluminum content of the dross, which dictates severe restrictions on the composition of the liquid fluxes.
In U.S. Pat. No. 5,716,426, Belen et al. describe a method to process aluminum dross and aluminum dross residue, essentially free of NaCl and KCl with lime at a temperature below 1600° C. to form at least partly melted calcium aluminate. The separation of the aluminum from the dross can be performed in the same vessel used to react the dross residue with the calcium oxygen compound. In addition to the limitations described above, particularly the fact that the dross residue must be essentially free from NaCl—KCl salts, these processes were verified to be ineffective in producing good quality, environmentally safe calcium aluminate products using contaminated NMP as confirmed by our own experiments described under Example 1.
For all these reasons, the production of high quality, completely pre-melted calcium aluminate using contaminated dross residue as a precursor of alumina is not presently possible at acceptable costs.
Accordingly, there is a real need in the aluminum and steel industries for a novel approach to produce high quality pre-melted calcium aluminate using contaminated NMP, in an environmentally responsible and cost effective manner.